Since plastic material of polymethyl methacrylate resin, polystyrene resin, polycarbonate resin, etc., has excellent impact resistance and transparency, and is light and easily processed, it is used in skylight material of buildings, windows for vehicles, lamp lenses, instrument covers, and the like as a material substitute for glass. However, since plastic material has poor surface properties in view of scratch resistance, chemical resistance, and weather resistance, compared to glass, the surface properties of the plastic material are modified. A method of coating heat-curable coating compositions such as polyorganosiloxane-based compositions and melamine-based compositions, and a method of coating polyfunctional acrylate-based active energy ray-curable coating compositions have been proposed as methods of modifying the surface properties of plastic material.
Regarding these methods, Patent Literature 1 and 2 disclose the inventions of coating compositions obtained by mixing poly(meth)acrylate of mono- or poly-pentaerythritol, urethane(meth)acrylate having at least two (meth)acryloyl groups per molecule, and poly[(meth)acryloyloxy alkyl](iso)cyanurate in a specific proportion. These coating compositions have excellent scratch resistance and weather resistance.
On the other hand, a method of adding inorganic material or inorganic-organic hybrid material to a coating composition is known as a method of improving the scratch resistance and weather resistance of a general coating composition. For example, Patent Literature 3 and 4 discloses a layered silicone polymer having a phyllosilicate mineral layer structure comprising a 2:1- or 1:1-type laminate having a sheet of tetrahedrons with a central atom of Si and a sheet of octahedrons with a central atom of a metal, and the layered silicone polymer having an organic group being covalently bonded to at least part of the silicone. Patent Literature 5 discloses a coating composition in which the layered silicone polymer of Patent Literature 3 or 4 is used as a filler.
Recently, in accordance with expansion of the outdoor application of plastic material, further improvement in the surface properties (scratch resistance and weather resistance) of plastic material is desired. However, the coating composition disclosed in Patent Literature 5 cannot form a cured coating film having excellent scratch resistance, weather resistance, and adhesion to a plastic substrate, all satisfying the aforementioned requirements. Further, even when a layered silicone polymer disclosed in Patent Literature 3 or 4 is added to a coating composition disclosed in Patent Literature 1 or 2, a coating composition capable of forming a cured coating film having excellent scratch resistance, weather resistance, and adhesion to a plastic substrate, all satisfying the aforementioned requirements, cannot be obtained.
To solve the problems of one-coat coating film obtained by using the aforementioned coating composition, Patent Literature 6 suggests a multi-layered coating film method using a specific undercoating composition. However, the method of Patent Literature 6 has problems such that the top coating composition mainly comprises an organic component, and the weather resistance and scratch resistance are insufficient compared to compositions in which inorganic material or inorganic-organic hybrid material disclosed in Patent Literature 3 to 5 is mixed.